How To Test Car Thermostat Without Removing It?

A vehicle thermostat is a crucial component of the cooling system. It helps transport coolant from the radiator to the engine to maintain optimal engine operation. 

It is a good practice to check your thermostat on every routine service to help regulate the coolant in the engine. The car thermostat relies on temperature changes to function properly.

For example, when you start your vehicle in the morning, the thermostat will not allow coolant to pass to the engine block until the engine reaches its normal operating temperature, permitting antifreeze to flow to the engine and radiator. 

If the thermostat goes bad, it’ll restrict coolant flow, leading to an engine overheating. Engine overheating can cause a blown head gasket or catastrophic issues like swapped pistons, cracked blocks, or damaged cylinder heads.

You don’t want any of this to happen. Therefore, you need to test your thermostat regularly and ensure it works properly. This article will discuss how to test a car thermostat without removing it.

test car thermostat without removing

How Can I Test My Thermostat Without Removing It?

If your thermostat is stuck open or closed, you may think the best option is to lose it and take it to a mechanic for diagnosis. I have good news. I will show you how to check for bad thermostats in a car without removing them.

One common cause of overheating in a car is a faulty thermostat. If you suspect this component, inspect it to see if it is the culprit. Here’s how to check a car thermostat without removing it. 

Check if the thermostat is stuck open.

Whenever a vehicle is overheating, the radiator hose should be hot. However, if your thermostat is stuck closed, the radiator hose will not be hot even when the engine is overheating. Therefore, the first thing to do is to examine the radiator hose.

Park the vehicle on solid and leveled ground. Engage the car to park or neutral, depending on your transmission. Start the vehicle and allow it to idle until the temp needle on the dashboard starts creeping towards the North Pole. 

Get a towel and hold the upper radiator hose. If it’s warm, you have a stuck open thermostat that’s not allowing the engine to reach its full operating temperature before sending coolant into the engine.

It’s okay to think you did not allow the engine to reach its normal operating temperature before checking the radiator hose. So, you can allow the engine to run for a little more time before checking the hose again. But, if it’s still warm, you have a stuck open thermostat that needs replacement.

Kindly note that thermostats open at about 185 degrees Fahrenheit. So, a healthy thermostat should remain closed until the coolant gets to this point. And the coolant should be very hot at 185 degrees Fahrenheit.

Check if the thermostat is stuck closed.

It’s rare to have a stuck closed thermostat, but it’s easy to identify because the vehicle will start overheating

Turn off the engine. Get a rag or towel and hold the upper radiator hose. The radiator hose should be extremely hot at this point. However, if the hose is cool, you have a stuck closed thermostat that needs replacement. 

Remember, a common stuck closed thermostat symptom is engine overheating. If the thermostat is stuck in a closed position, it won’t allow coolant circulation to the engine and the radiator. So, do not allow the radiator to overheat.

If you find out the thermostat is bad, you have just one option – car thermostat replacement.

If you’re wondering how to tell if a thermostat is bad or how to unstick a thermostat in a car, read this well-detailed article on the causes, symptoms, and how to fix a bad thermostat.

benefits of removing thermostat

What happens if a thermostat is stuck closed?

Normally, a vehicle thermostat remains closed when you start a vehicle until it reaches the normal operating temperature. This means it will not allow coolant to flow from the radiator to the engine. However, once the engine reaches its full operating temp, it’ll open and grant coolant passage to cool the engine.

However, if the thermostat is stuck closed, it’ll prevent coolant passage even when the engine is extremely hot. If you have a stuck closed thermostat, the upper and lower radiator hose will be cool even when the engine is overheating. 

The upper and lower radiator hoses should be hot when an engine reaches normal operating temperature. In any case, the upper Hose should be hotter than the lower hose. You have a stuck closed thermostat if the radiator hoses are cool after a 5-15 minute run.

Can you run a car without a thermostat in the summer?

Yes, you can drive a car without a thermostat in both summer and winter. The primary function of a thermostat is to allow coolant passage when the engine gets to 185 degrees Fahrenheit. Without the thermostat, there will be constant coolant flow from the radiator to the engine.

In any case, driving without a thermostat is not ideal here in the United States. In some African countries, the service technicians don’t replace thermostats once it goes bad.

You’ll enjoy the benefits of removing a thermostat (which is a no-closed position) if you drive without this component. 

Why is my car overheating with a new thermostat?

Several factors can cause a car to overheat. So, suppose your vehicle is overheating with a new thermostat. It could be you have a bad water pump, bad radiator, lousy cooling fan, clogged heater core, or any cooling system-related issues. 

If your new thermostat is not opening, it could be that you didn’t install it properly. The most important thing to do is park the vehicle in your garage until you fix the problem. If you ignore and continue driving with it, it can escalate and cause catastrophic damage. 

Can you drive with a thermostat stuck open?

While a stuck closed thermostat can cause engine overheating, leading to a catastrophic breakdown if you fail to fix it on time, you can drive thousands of miles with a stuck open thermostat. 

However, you may experience some issues along the way. It is expected for coolant to continue circulating coolant from the radiator to the engine and back when the engine is working at an average temperature. This circulation process poses a problem when the engine is cold.

The ECM will keep the engine on a warm cycle to get the engine to a higher operating temp, burning more fuel. In any case, the amount of extra fuel burnt can be as little as insignificant.

Final Words 

A car thermostat can fail because of dirt, debris, or high-temperature differences. The common symptoms you’ll notice are overheating engine warning lights and coolant leaks. I recommend you check out this article – Coolant Temperature Reading; what’s Normal and what’s Not.

So, if your engine overheats and you suspect a lousy thermostat to be the culprit, inspect the component. Thankfully, you can test a car thermostat without removing it.

Scroll up to the simplified guide above and follow the instructions to check your thermostat. 

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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